This invention relates to an improved process for producing molded thermoplastic resins.
In the art of extrusion or compression or molding of a thermoplastic resin, it is known that flowability of the resin in the die can be improved by lubricating the inner surface of the die with a lubricant. This is taught with respect to extrusion molding, in U.S. Pat. Nos. 2,597,553 and 2,688,153. By coating the inner surface of a die with a lubricant, flowability of a thermoplastic resin within the die can be markedly improved. As a result, molding at a lower pressure is possible.
Attempting to lubricate the inner surface of a die with a lubricant, however, poses various problems. The greatest problem is that it is difficult to apply the desired thickness of the lubricant uniformly on the inner surface of a die during molding. Consequently, at the portion of the die where the lubricant is applied in too large a quantity, the resin flow will be accelerated, thereby making uniform molding impossible.
Roughening of the inner surface of the die has been suggested as a possible solution to this problem. See U.S. Pat. No. 4,087,222. This approach, however, has not been altogether satisfactory.
U.S. Pat. No. 4,235,834 discloses biaxially oriented molding of a thermoplastic resin under extrusion pressure in a die lubricated on its inner surface with a lubricant. Difficulty in uniformly applying the lubricant on the inner surface of the die during molding, however, causes localized failures of lubricant film which make it difficult to obtain a uniform biaxially oriented molded product. Further, because of the difficulty in uniformly applying a constant thickness of the lubricant, it is difficult to achieve a high degree of stretching. Indeed, only stretching from about 30% to about 120% in each direction can be achieved. In other words, only stretching to an area ration of 1.69 to 4.84-fold is usually possible. Stretching to a 5-fold area ratio or higher is generally not possible.
U.S. Pat. No. 3,632,841 discloses compression molding in a die lubricated on its inner surface with a lubricant. Here too, however, it is diffucult to apply the lubricant uniformly over the inner surface of a die during molding. Consequently, uniform molding is difficult. In order to overcome that problem, U.S. Pat. No. 3,632,841 requires the molding be performed under a low compression at a low stretching rate. In U.S. Pat. No. 3,632,841 biaxially oriented compression molding of an acrylic resin using a lubricant is effected at a low pressing pressure of 300 to 600 psi and at a low lateral flow rate of 0.1 to 72 in./min. Pressing must be conducted under such a low pressure and at such a low flow rate in order to prevent cracking due to poor lubrication on the inner surface of the die. That is, when the pressing pressure is increased, the frictional force between the acrylic resin and the die surface is increased in proportion to the pressing pressure, thereby causing cracking.
It would be highly desirable to improve the lubricating characteristics of the die surface so that good biaxially oriented molding may be performed and a highly oriented molded product may be molded satisfactorily even when molded under a high compression force and at a high stretching rate.
Another problem with prior art processes for extrusion or compression molding with a lubricant is the difficulty in removing the lubricant. There is no simple washing method for removing the lubricant. Moreover, if washing is performed with a large amount of washing liquor, a considerable cost will be incurred in the disposal of liquor.